This is where the wealth of SRB2 levels are stored. They are seperated into groups to make it easier to find what you want.

Single Player - Level Packs
These are single player level packs, which contain files that have a lot more than one level in them. These are often WADs that add a new quest into the game, or add a few new zones.

Multiplayer - Level Packs
These are multiplayer level packs, which contain files that have a lot more than one level in them. These are often custom deathmatch rotations, and level design contest WADs.

Single Player Levels
These are one player stages that require the player to find the end of the stage or beat bosses. They are for Single Player mode, but most can also be played in Cooperative or Race mode. They can also occasionally involve finding hidden emeralds or simply racing to the end of the level.

Match Levels
These levels are primarily for the Match and Tag modes in SRB2 multiplayer. They have no exit, and are arenas for beating on each other.